Molybdate-containing corrosion inhibitors

ABSTRACT

A process for inhibiting the corrosion of metals in contact with aqueous systems is provided which process comprises adding to such systems an effective amount of a water treatment composition comprising a source of molybdate ion and a water-soluble polymer containing pendant amide functionality.

This is a continuation of application Ser. No. 07/168,913, filed on Mar.15, 1988 abandoned.

TECHNICAL FIELD OF THE INVENTION

The present invention is in the technical field of corrosion inhibitorsfor metals in contact with aqueous systems and in particular corrosioninhibitors useful in industrial cooling water systems. The presentinvention is particularly useful in industrial recirculating coolingwater systems.

BACKGROUND OF THE INVENTION

Compositions useful for inhibiting the corrosion of metals in contactwith aqueous systems, such as corrosion inhibitors used in industrialcooling water systems, often contain zinc salts, such as zinc chloride,zinc sulfate, zinc acetate, or the like, which compounds provide, upondissolution in aqueous systems, the zinc cation to the system. Inindustrial cooling water systems, and other systems in which corrosioninhibiting compositions are used, the waters employed are ofteneventually discharged as effluent, and upon such discharge the corrosioninhibitors incorporated therein or by-products of spent corrosioninhibitor systems, may reach natural water systems, such as rivers andlakes and the like. Since zinc compounds generally are toxic to aquaticlife, such as fish, it is desirable to minimize the level of zinccompounds in any such effluents, and possibly eliminate zinc compoundscompletely. Hence it is desirable to provide a process for inhibitingthe corrosion of metals in contact with aqueous systems, and compositionfor such process, which is both effective as a corrosion inhibitor andcontains little or no toxic compounds. It is desirable to provide such aprocess and composition that is effective in industrial aqueous systemsemploying significant amounts of water, particularly those systemswherein the water employed is eventually discharged as effluent. It isdesirable to provide such a process and composition that is effective inindustrial cooling water systems, and in particular industrialrecirculating cooling water systems, and also controls scale deposits.

DISCLOSURE OF THE INVENTION

The present invention provides a process for inhibiting corrosion ofmetals in contact with aqueous systems comprising the addition to thewater of such aqueous systems an effective amount of a water treatmentcomposition comprising a source of molybdate ion and a water-solublepolymer(s) containing pendant amide functionality, such polymers beingprimarily derived from acrylamide and alkyl substituted acrylamidecontaining copolymers/terpolymers with acrylic acid and/or its homologssuch as methacrylic acid and the like. The present invention alsoprovides such a composition for water treatment.

PREFERRED EMBODIMENTS OF THE INVENTION

The process of the present invention is directed to the inhibition ofcorrosion of metals in contact with aqueous systems, and in preferredembodiment is directed to the inhibition of corrosion of metals incontact with cooling water systems. In more preferred embodiment theprocess is directed to the inhibition of corrosion of metals in contactwith recirculating water systems, particularly industrial recirculatingwater systems, such as industrial recirculating cooling water systems.In this process the water treatment composition is at least added ineffective amount to the waters of such systems, and particularly withrespect to recirculating water systems is preferably maintained at aneffective level within said system. The process inhibits or retardscorrosion of metal(s) in contact with the water of such systems, andretards or diminishes the formation of scale deposits within suchsystems.

The water treatment composition includes a source of molybdate ion,i.e., MoO₄ ⁻², preferably an alkali metal salt of molybdate, such assodium molybdate, although other sources, such as molybdic acid, may beused. It is believed that this component in the present composition hasas its active form the oxy anion MoO₄ ⁻² and, regardless of themechanism of the activity of the present composition, it is believedthat the present composition may be precisely defined, as to the sourceof molybdate ion, in terms of the molybdate ion level provided by suchsource than the amount of such source utilized.

The water treatment composition further includes a water-solublepolymer, or mixture of polymers, containing pendant amidefunctiontionality, primarily derived from copolymers/terpolymers ofacrylamide and/or alkyl substituted acrylamide with acrylic acid and/orits homologs such as methacrylic acid and the like. The pendant amidefunctionality of such water-soluble polymer may have the generalstructure of Formula I: ##STR1## wherein the carbonyl carbon is bondedto the polymer backbone and wherein R and R₁ are independently H oralkyl having 1 to 6 carbons, wherein such N-substituted alkyl may bebranched or straight chain, and in preferred embodiment one of R and R₁is H and the other is alkyl. In preferred embodiment the water-solublepolymer is a copolymer or terpolymer of from about 25 to 95 mole percent(meth)acrylic acid and from about 5 to 75 mole percent alkyl substitutedacrylamide, particularly wherein such alkyl substituted acrylamideprovides the pendant amide functionality of Formula I, and moreparticularly wherein one of R and R₁ is alkyl having from 1 to 6carbons, and the other is H. In a particularly preferred embodiment, thepolymer is a copolymer or terpolymer of from 25 to 95 mole percent(meth)acrylic acid and from about 5 to 75 mole percent tertiary butylacrylamide.

The polymer component of the present water treatment composition ispreferably a polymer as described above having a molecular weight offrom about 500 to about 100,000, and in further preferred embodiment,particularly when the polymer is a copolymer or terpolymer of(meth)acrylic acid and tertiary butyl acrylamide, and more particularlywhen such a copolymer or terpolymer within the mole percentage rangesdescribed above for such combination, the molecular weight thereof isfrom about 500 to 25,000, and more particularly from 10,000 to 20,000.

A particularly useful polymer is a terpolymer of acrylic acid,methacrylic acid, and alkyl substituted acrylamide, in particular suchterpolymer of from 25 to 90 mole percent of the acrylic acid andmethacrylic acid taken together and from 5 to 75 mole percent of thealkyl substituted acrylamide. In this embodiment, a preferred terpolymeris one derived in pertinent part from an alkyl substituted acrylamideproviding the pendant amide functionality of the general Formula I abovewherein only one of R and R₁ are alkyl, the other being hydrogen (H),and such alkyl having from 1 to 6 carbons.

In further preferred embodiment the alkyl substituted acrylamide monomerfrom which the polymer is derived is one in which the alkyl substituenthas from 1 to 4 carbons, such as methyl acrylamide, ethyl acrylamide,propyl acrylamide, isopropyl acrylamide, n-butyl acrylamide, t-butylacrylamide, and the like, and a co- or terpolymer of acrylic acid,methacrylic acid, and t-butyl acrylamide having from 40 to 80 molepercent acrylic acid, 0 or from 5 to 40 mole percent methacrylic acid,and from 5 to 40 mole percent t-butyl acrylamide, has been foundparticularly useful in the water treatment composition, particularlywhen such co- or terpolymer has a molecular weight of from 500 to25,000.

The polymer component of the present water treatment composition isbelieved active as a dispersant, stabilizing calcium carbonate in watersystems. For corrosion inhibition with concommitant retardation of scaledeposits in aqueous systems dispersant-type polymers commonly areemployed in combination with zinc compounds. As discussed above, zinccompounds heretofore used in corrosion inhibition formulations may betoo toxic to aquatic life, particularly fish, to be utilized in certainindustrial applications where the volume of zinc-containing effluent andthe level of zinc within that effluent results in too high of a level ofzinc compounds reaching natural water systems. Molybdate compounds,however, are believed of sufficiently low toxicity to aquatic life thatin the amounts employed in the process of the present invention,including industrial recirculating cooling water corrosion inhibitionprocess, that the molybdate present in the discharged effluent poses notoxic danger in natural water systems.

The water treatment composition of the present invention mayadvantageously include other components such as organic phosphonates,water-soluble orthophosphates, azoles such as tolytriazoles andmercaptobenzothiazoles, polycarboxylic acids, and other agents that mayprovide corrosion inhibition or anti-scale activity or supplement thewater treatment composition by providing stabilization for one or moreof its components. Such organic phosphonates include organo-phosphonicacids and water soluble salts thereof, such as the alkali metal ammoniumsalts, and phosphono-carboxylic acids including(poly)phosphono(poly)carboxylic acids, and aminoalkylene phosphonicacids. Specific examples of such organic phosphonates include1,1-ethylidenediphosphonic acid, 1-hydroxyethylidene-1,1-diphosphonicacid, butylidene diphosphonic acid, 1-aminoethylidene-1,1-diphosphonicacid, amino tri(methylene)phosphonic acid, 2-phosphonobutane1,2,4-tricarboxylic acid, and the like. Of the foregoing, it has beenfound that 1-hydroxy ethylidene-1,1-diphosphonic acid (referred toherein as HEDP) and 2-phosphonobutane 1,2,4-tricarboxylic acid (referredto herein as PBTC) are particularly useful components of the presentwater treatment composition. The water-soluble orthophosphates aresources of the orthophosphate ion (PO₄ ⁻³) and include phosphoric acid,simple orthophosphate salts, and other compositions that provide thedesired level of orthophosphate ion under given use conditions. It isbelieved that the invention, in any embodiment wherein water-solubleorthophosphates are included in the water treatment composition, may bereasonably defined when such source of orthophosphates is defined interms of the level of level of orthophosphate ion provided thereby.Particularly useful polycarboxylic acids include the long chain diacidsderived from fatty acids such as diacids having a molecular weight offrom about 200 to about 1,000. In addition to the foregoing corrosioninhibitors or anti-scale agents, or stabilizers, the water treatmentcomposition of the present invention may include of course suitablesolvents or diluents or carriers.

The amount of water treatment composition that is effective ininhibiting corrosion of metals in aqueous systems will vary depending ona number of factors including the type of metal(s) to be protected andthe water conditions. In general corrosion inhibition activity in somesystems may be provided with as little as 0.5 ppm of molybdate ioncombined with 0.5 ppm of the polymer described above, and as a practicalupper limit the amounts of these components generally would not need toexceed 200 ppm each. A particularly useful level of water treatmentcomposition, either as a dosage or particularly for recirculatingsystems as a maintenance level, is from 5 to 150 ppm of molybdate ionand from 5 to 150 ppm of the polymer, with from 10 to 100 ppm of eachcomponent being preferred. When additional components are added to thewater treatment composition, they may be used at levels similar to thatfor the molybdate ion and polymer or at lower levels. For instance, HEDPor PBTC may be included at lower or greater levels than the molybdateion and the polymer taken alone or together, while the azoles andpolycarboxylic acids typically, although not necessarily, are includedin amounts less than the molybdate ion and the polymer taken alone.

The present water treatment composition is particularly useful inaqueous systems maintained at a pH of from about 7.5 to about 9.0, andat a water temperature of from about 80° to 150° F.

The present water treatment composition in preferred embodiment is onein which no source of zinc ion is employed.

POLYMER

In the following Examples 1 through 6 the term "polymer" used thereinrefers to a terpolymer of acrylic acid/methacrylic acid/t-butylacrylamide in respective mole ratios of about 60/20/20 and having amolecular weight of about 14,600.

EXAMPLE 1

Seven-day container tests were conducted to compare corrosion rates inthe presence of varying levels of molybdate ion, polymer, and HEDP(hydroxyethylidene diphosphonic acid). In these tests coupons were heldimmersed in the test water, under constant agitation, for a period ofseven days. The test water was held at a pH of 8.5 and had a totalalkalinity of 90 ppm (as CaCO₃), 70 ppm calcium as CaCO₃ and 35 ppmmagnesium as CaCO₃. Corrosion rates, in mpy (mils per year) weredetermined for three types of coupons, i.e., copper, admiralty brass,and mild steel. When the copper and admiralty brass coupons were tested,7.0 ppm tolytriazole was added to the test water. The corrosion rateswere calculated based on coupon weight loss after removal of deposits.The results are shown below in Table I.

                                      TABLE I                                     __________________________________________________________________________                              Corrosion Rate (mpy)                                Tests                                                                            MoO.sub.4 .sup.-2 (ppm)                                                               Polymer (ppm)                                                                         HEDP (ppm)                                                                           Copper                                                                            Adm. brass                                                                          mild steel                                __________________________________________________________________________    a  none    none    none   0.01                                                                              0.08  18.3                                      b  none    100     none   0.31                                                                              0.12  16.10                                     c  100     none    none   0.01                                                                              0.01  1.60                                      d   50      50     none   0.01                                                                              0.01  1.00                                      e  100     100     none   0.03                                                                              0.03  0.50                                      f  none     50      50    0.30                                                                              0.14  3.37                                      g  none    none    100    0.01                                                                              0.01  1.50                                      h  none    100     100    0.06                                                                              0.16  0.29                                      i  100     none    100    0.01                                                                              0.10  0.10                                      j   50     none     50    0.01                                                                              0.01  0.04                                      k   50      50      50    0.20                                                                              0.18  0.61                                      l  100      50      50    0.07                                                                              0.12  0.08                                      m   50     100      50    0.10                                                                              0.06  0.06                                      n   50      50     100    0.01                                                                              0.15  0.41                                      o  100     100     100    0.06                                                                              0.06  0.17                                      __________________________________________________________________________

EXAMPLES 2 and 3

One-day container tests were conducted to compare corrosion rates atvarying pH's for two water treatment compositions. In these testscoupons were held immersed in the test water for a one day (24 hour)period with constant agitation. The test water contained 175 ppm calciumand 87 ppm magnesium, and was held at a temperature of about 50° C. Theresults are reported simply in milligrams of metal loss. Each watertreatment composition provided a use level of 10 ppm MoO₄ ⁻² (from Na₂MoO₄), 15 ppm ortho-PO₂ ⁻³ (from H₃ PO₄), and 5 ppm of the polymerdescribed in the text above. In Example 2 the composition furtherincluded 5 ppm of PBTC (2-phosphonobutane-1,2,4-tricarboxylic acid). InExample 3 the composition further included 5 ppm HEDP (hydroxyethylidenediphosphonic acid). The metal loss versus pH results are set out belowin Table II.

                  TABLE II                                                        ______________________________________                                                Metal Loss (ppm)                                                      Example   pH 7.5  pH 8.0     pH 8.5                                                                              pH 9.0                                     ______________________________________                                        blank     88.3    75.8       86.6  99.3                                       2         10.4    13.2       13.0  27.0                                       3         9.2     6.0        7.7   23.5                                       ______________________________________                                    

PILOT COOLING TOWER TEST

The pilot cooling tower embodies the features of a standard coolingtower and hence permits a determination of water-treatment performanceunder simulated conditions. The pilot cooling tower also is equipped tocontrol the various factors that have an affect on corrosion rate, suchas water composition, velocity, water temperature and pH and the like.In general, the cooling water from a basin flows over eight heattransfer tubes in series and then through a conduit in which is held thetest specimens, tubes and coupons, after which it returns to the towersection where it is sprayed over a film-type packing above the basin.The tower section is provided with an upper fan that is thermostaticallycontrolled based on the basin water temperature. The basin has feedinlets for the make-up water, the pH control solution (0.07N H₂ SO₄),and the treatment chemicals, plus an outlet and pump for the blowdown.Between the basin and heat transfer tubes the pilot cooling tower isequipped with a centrifugal circulation pump and pH and conductivitycells. A flow meter is disposed on the line between the heat transfertubes and the test specimens. The test conditions used are set forth inTable III below. Any variations from these conditions is noted in thespecific examples following.

                  TABLE III                                                       ______________________________________                                        Basin water temperature                                                                         100° F.                                              Return water temperature                                                                        110° F.                                              pH                7.5 to 9.5                                                  Conductivity      300 to 8,000 μmhos                                       flow rate         2.0 gallons per minute (gpm)                                flow velocity     3.0 feet per second (ft/s)                                  HTI               24 hours                                                    Test duration     14 days                                                     Cycles of concentration                                                                         4                                                           Make-up Water (ppm)                                                           CaH               90                                                          MgH               50                                                          Na                40                                                          "M" alkalinity    110                                                         Cl                63 to 64                                                    SO.sub.4          48                                                          SiO.sub.2         0                                                           ______________________________________                                    

The performance of the water treatment used is monitored both by depositweight and corrosion rate, the latter of which is determined by specimenweight loss after deposit removal.

EXAMPLE 4

Using the Pilot Cooling Tower Test described above, with an actual pHrange of 7.5 to 7.9, a water treatment composition was tested, whichtreatment comprised the following:

    ______________________________________                                        Component             Use level (ppm)                                         ______________________________________                                        MoO.sub.4 .sup.-2 from Na.sub.2 MoO.sub.4.2H.sub.2 O                                                8.0 to 12.0                                             ortho PO.sub.4 .sup.-3 from H.sub.3 PO.sub.4                                                        8.0 to 12.0                                             PBTC                  2.0 to 3.0                                              tolyltriazole         2.2 to 4.8                                              diacid                0.7 to 1.5                                              polymer               7.0 to 15.0                                             ______________________________________                                    

The diacid used was a fatty acid for stabilization of the tolyltriazoleand is such as such subsequently. Both tubes and coupons were used astest specimens. The deposit weights and corrosion rates for the varioustest specimens are set out below in Table IV.

                  TABLE IV                                                        ______________________________________                                        Type of Test   Deposit Weight                                                                             Corrosion Rate                                    Specimen       (g)          (mpy)                                             ______________________________________                                        Admiralty brass tube                                                                         16.5         0.228                                             Admiralty brass tube                                                                         21.0         0.219                                             Mild steel tube                                                                              311.8        2.156                                             Mild steel tube                                                                              354.0        2.391                                             Mild steel tube                                                                              374.0        2.246                                             Stainless steel tube                                                                         5.2          0.001                                             Admiralty brass coupon                                                                       2.4          0.229                                             Mild steel coupon                                                                            30.6         3.714                                             ______________________________________                                    

EXAMPLE 5

The Pilot Cooling Tower Test described in Example 4 above was repeatedexcept the actual pH range was 8.4 to 8.8 and the use levels of thewater treatment composition components changed to the following:

    ______________________________________                                        Component           Use level (ppm)                                           ______________________________________                                        MoO.sub.4 .sup.-2   4.0 to 8.0                                                ortho PO.sub.4 .sup.-3                                                                            4.0 to 8.0                                                PBTC                1.0 to 2.0                                                tolyltriazole       2.4 to 4.8                                                diacid              0.75 to 1.5                                               polymer              7.5 to 15.0                                              ______________________________________                                    

In this test the components and source thereof are as described inExample 4 above. The type of test specimens, deposit weights, andcorrosion rates for this test are set out below in Table V.

                  TABLE V                                                         ______________________________________                                        Type of Test   Deposit Weight                                                                             Corrosion Rate                                    Specimen       (g)          (mpy)                                             ______________________________________                                        Admiralty brass tube                                                                         42.5         0.341                                             Admiralty brass tube                                                                         47.0         0.283                                             Stainless steel tube                                                                         19.4         0.000                                             Mild steel tube                                                                              266.8        1.415                                             Mild steel tube                                                                              303.0        1.611                                             Mild steel tube                                                                              191.4        1.012                                             Admiralty brass coupon                                                                       2.8          0.194                                             Mild steel coupon                                                                            21.1         2.071                                             ______________________________________                                    

EXAMPLE 6

The Pilot Cooling Tower Test described in Example 4 above was againrepeated except the actual pH range was 7.9 to 8.5 and the basin andreturn water temperatures were respectively 120° and 130° F., and theuse levels of the water treatment composition changed to the following:

    ______________________________________                                        Component           Use level (ppm)                                           ______________________________________                                        MoO.sub.4 .sup.-2   5.0 to 10.0                                               ortho PO.sub.4 .sup.-3                                                                            12.0 to 24.0                                              PBTC                6.0 to 12.0                                               tolyltriazole       2.4 to 4.8                                                diacid              0.75 to 1.5                                               polymer             7.5 to 15.0                                               ______________________________________                                    

In this test the components and source thereof are as described inExample above. The type of test specimens, deposit weights, andcorrosion rates for this test are set out below in Table VI.

                  TABLE IV                                                        ______________________________________                                        Type of Test   Deposit Weight                                                                             Corrosion Rate                                    Specimen       (g)          (mpy)                                             ______________________________________                                        Admiralty brass tube                                                                         31.0         0.254                                             Admiralty brass tube                                                                         48.0         0.341                                             Stainless steel tube                                                                         14.0         0.000                                             Mild steel tube                                                                              354.6        2.176                                             Mild steel tube                                                                              368.4        2.268                                             Mild steel tube                                                                              322.2        1.552                                             Admiralty brass coupon                                                                       6.5          0.430                                             Mild steel coupon                                                                            23.1         1.987                                             ______________________________________                                    

INDUSTRIAL APPLICABILITY OF THE INVENTION

The present invention is applicable to industries employing aqueoussystems in contact with metal(s) subject to corrosion, and in particularto the cooling water industries, and more particularly to recirculatingcooling water industries.

We claim:
 1. A water treatment composition for the inhibition ofcorrosion in metals in contact with aqueous systems consisting of:awater-soluble source of molybdate ion; and a water-soluble polymercontaining pendant amide functionality, said pendant amide functionalityhaving the structure of ##STR2## wherein the carbonyl carbon is bondedto the backbone of said water-soluble polymer and R and R₁ areindependently H or alkyl having 1 to 6 carbons, provided that not bothof R and R₁ are H.
 2. The water treatment composition of claim 1 whereinsaid water-soluble polymer is a co- or terpolymer of from 25 to 95 molepercent (meth)acrylic acid and from 5 to 75 mole percent alkylsubstituted acrylamide, said alkyl substituted acrylamide providing thependant amide functionality.
 3. The water treatment composition of claim2 wherein said water-soluble polymer is a terpolymer of from 40 to 80mole percent acrylic acid, from 5 to 40 mole percent methacrylic acid,and from 5 to 40 mole percent tertiary butyl acrylamide.
 4. The watertreatment composition of claim 1 wherein said water-soluble polymer hasa molecular weight of from 500 to 100,000.
 5. The water treatmentcomposition of claim 2 wherein said water-soluble polymer has amolecular weight of from 500 to 25,000.
 6. The water treatmentcomposition of claim 1 wherein said source of molybdate ion and saidwater-soluble polymer are present in sufficient relative amounts toprovide from 0.5 to 200 ppm molybdate ion and from 0.5 to 200 ppm ofsaid water-soluble polymer when added to an aqueous system.
 7. A processfor inhibiting corrosion of metals in contact with aqueous systemscomprising the addition to the water of such system an effective amountof a water treatment composition as defined in claim
 1. 8. The processof claim 7 wherein said water treatment composition is as defined inclaim
 2. 9. The process of claim 8 wherein said water treatmentcomposition is as defined in claim
 3. 10. The process of claim 7 whereinsaid water treatment composition is as defined in claim
 4. 11. Theprocess of claim 8 wherein said water treatment composition is asdefined in claim
 5. 12. The process of claim 7 wherein sufficient watertreatment composition is added to maintain a level of from 0.5 to 200ppm molybdate ion and from 0.5 to 200 ppm of said water-soluble polymerin said aqueous system.
 13. The process of claim 12 wherein sufficientwater treatment composition is added to maintain a level of from 5.0 to150 ppm molybdate ion and from 5 to 150 ppm of said water-solublepolymer in said aqueous system.
 14. The process of claim 13 whereinsufficient water treatment composition is added to maintain a level offrom 10 to 100 ppm molybdate ion and from 10 to 100 ppm of saidwater-soluble polymer in said aqueous system.
 15. The process of claim12 wherein the pH of said aqueous system is maintained at from 7.5 to9.0.
 16. The process of claim 12 wherein the temperature of the water insaid aqueous system is maintained between 80° to 150° F.
 17. A watertreatment composition for the inhibition of corrosion in metals incontact with aqueous systems consisting essentially of:a water-solublesource of molybdate ion; a water-soluble co- or terpolymer of from 25 to95 mole percent (meth)acrylic acid and from 5 to 75 mole percent alkylsubstituted acrylamide, said alkyl substituted acrylamide being anacrylamide unit substituted at the amide nitrogen with alkyl having fromone to six carbons, and said water-soluble co- or terpolymer having amolecular weight of from 500 to 100,000; and at least one of an organicphosphonate, a source of orthophosphate ion, and an azole.
 18. The watertreatment composition of claim 17 wherein said water-soluble co- orterpolymer is a terpolymer of from about 25 to 90 mole percent acrylicand methacrylic acid and from 5 to 75 mole percent tertiary butylacrylamide.
 19. The water treatment composition of claim 17 wherein saidwater-soluble co- or terpolymer has a molecular weight of from 500 to25,000.
 20. A process for inhibiting corrosion of metals in contact withaqueous system comprising the addition to the water of such system aneffective amount of a water treatment composition as defined in claim17.
 21. The process of claim 20 wherein the pH of said aqueous system ismaintained at from 7.5 to 9.0.
 22. The process of claim 20 wherein thetemperature of the water in said aqueous system is maintained between80° to 150° F.
 23. The water treatment composition of claim 17consisting essentially of said water-soluble source of molybdate ion,said water-soluble co- or terpolymer, and said source of orthophosphateion.